3.2 Mechanical Properties
3.2.1 Compressive Strength
BOF
(Moon, Yoo, & Kim, 2002) - converter slag --- 16.80%
(Geisler, et al., 2001) --- 22%
Table 8: Abrasion values (%) of steel slag.
3.2 Mechanical Properties
Concrete made with steel slag as an aggregate must have mechanical properties that are equal to or greater than that of ordinary portland cement concrete. It is important that these properties are not compromised by the replacement of natural aggregates with steel slag. The mechanical properties discussed in this section are compressive strength, tensile strength, flexural strength, elastic modulus and the water penetration of concrete containing steel slag.
3.2.1 Compressive Strength
When evaluating the compressive strength of portland concrete cement, specimens are subjected to various types of tests. In these tests, the specimens are loaded until failure occurs using a uniaxial compressive load. The compression test consists of
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loading the cylindrical concrete specimens in uniaxial compression and is accomplished by following the procedures of ASTM C 39. The failure of the specimen in compression can occur along one of three failure planes; through the cement paste, along the interface of the cement paste and the aggregate, or directly through the aggregate. The aggregates themselves are generally stronger then the cement paste, so they are of less importance in normal strength concrete but can become a greater factor in high strength concrete due to the relatively high strength of the cement paste relative to the strength of the aggregate (Mindess et al., 2003).
Most chemical admixtures have little effect on the strength of the concrete unless they are water-reducing super plasticizers, which can provide an increase in the strength from the greater cement hydration. This results from the improved dispersion of the cement particles and the elimination of large pores that can act as flaws on the internal structure (Mindess et al., 2003). Air entraining admixtures can affect the strength of the concrete since they can effect the water to cement ratio which can effect the porosity of the concrete. For concrete designed for workability with the addition of an air entraining admixture, the concrete strength might actually increase due to the low cement content (Mindess et al., 2003).
The compressive strength of concrete that contains steel slag as an aggregate has been reported on in many studies. These studies have examined the compressive strength with different percentages of steel slags as a replacement of the natural aggregates. The EAF slag used in this study by Manso et al. (2006) was weathered, homogenized through periodic over turning of the pile of slag for a minimum period of 90 days. The water to cement ratio was less than or equal to 0.6 and no admixtures were used in this study. The
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28 day compressive strength of concrete cubes ranged from 3000 psi (20.6 MPa) for specimens that contained 100% fine and 100% coarse EAF slag aggregate to 5100 psi (35.3 MPa) strength for specimens that contained 50% limestone and 50% steel slag as fine aggregate and 100% steel slag coarse aggregate. The control specimen which contained un unspecified limestone aggregate reached a compressive strength of 5600 psi (38.5 MPa). No correleation between the amount of steel slag in the concrete and compression strength could be seen from the results of this study.
In conducted by Polanco et al., (2011), the compressive strengths of concrete containing various amounts both EAF slag and ladle furnace slag as a fine aggregate were investigated. The EAF slag was used for 100% of the coarse aggregate in all test specimens. The water to cement ratio ranged from 0.6 for the control specimen which contained 25% limestone sand fine aggregate and 25% EAF slag fine aggregate (by total weight of the concrete) to 0.8 for the specimen with 25% EAF fine aggregate and 25%
LFS fine aggregate (by total weight). The 28 day compressive strength of the control reached a strength of 6750 psi (46.6 MPa) while a specimen which contained 37% EAF fine aggregate and 25% LFS fine aggregate reached a strength of 8700 psi (60.4 MPa).
The lowest compressive strength was 4570 psi (31.5 MPa) for concrete which contained fine aggregate in quantities of 25% for each of the EAF and LFS. This mixture had a w/c of 0.8. The results of this study showed that as the percentage of EAF and LFS increased, the compression strength also increased.
Comparable results were also achieved by Maslehuddin et al. (2003). In this study the 28 day compressive strength ranged from 4500 psi (31.4 MPa) with 45% coarse steel slag aggregate to 6200 psi (42.7 MPa) with 65% steel slag aggregate. While results
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vary slightly, the compressive strengths of the concretes prepared with the steel slag increased as the percentage of slag increased and were comparable to those prepared without the slag.
In work conducted at Cleveland State University (Obratil et al, 2008 and Obratil et al 2009), the compressive strength of concrete mixtures containing EAF slag as a replacement for coarse aggregate was studied and compared to ODOT Class C specification 499.03. The minimum compressive strength allowed by the ODOT specifications is 4000 psi (28.0 MPa) at 28 days. In the 2008 study various percentages of the coarse and the fine natural aggregate were replaced with steel slag to study the effects on the compressive strength. The compressive strength for the specimes with the replacement of the 0.75 inch (19mm) aggregate had an average compressive strength that ranged from 4200 psi to 5500 psi (29 MPa to 38 MPa) at 28 days. The specimens with the 0.5 inch (12.5mm) steel slag aggregate as a replacement had an average compressive strength of 4300 psi to 5600 psi (29.6 MPa to 38.6 MPa) at 28 days. When the “C” fines were replaced in the concrete the average compressive strength ranged from 3300 psi (22.8 MPa) for a 10 % replacement to 5400 psi (37.2 MPa) for 40% replacement at 28 days. All the specimens were able to meet the minimum ODOT specification for Class C paving concrete with the exception of the 10% replacement of the “C” fines. The 0.75 inch (19mm) aggregate showed no trends relating the percentage of slag aggregate to the compressive strength while the compressive strength decreased as the amount of 0.5 inch (12.5mm) slag aggregates increased. In the study of compressive strength for the “C”
fines the percentage replaced ranged from 10% to 40% and the strength increased as the percentage of slag increased.
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The specimens in the 2009 study at Cleveland State University contained steel slag ranging from 0% to 100% replacement of the coarse natural aggregate. All of the specimens containing the steel slag were able to achieve a compressive strength greater then the minimum specified by ODOT at 28 days. The control specimen reached a compressive strength of aproximately 5600 psi (38.6 MPa) at 28 days and the specimens with the slag ranged from 4200 to 5100 psi (29 to 35.2 MPa). This study showed that the ODOT specification for Class C concrete paving mixures could be obtained when steel slag is used as a replacement for natural coarse aggregates. The maximum compressive strength of the steel slag concrete was achieved when 50% of the natural coarse aggregate was used, but no correleation could be observed between the percentage used and the compression strength obtained.
High strength concrete is considered to have a compressive strength greater than 6000 psi (41 MPa) and researchers have found that the hardened strength-limiting paste and the transition zone are no longer the limiting factor, but that it is the mineralogy and strength of the coarse aggregate that will control the ultimate strength of the concrete.
A high strength concrete containing steel slag as a coarse aggregate in percentages of 25% and 100% resulted in compression strengths of 47.1 and 54.1 MPa (6800 and 7850 psi) which were both higher then the control compression strength of 39.3 MPa (5700 psi). The water to cement ratio was 0.5, and it could be seen from this study that the compression strength increased as the percentage of steel slag increased (Etxeberria et al., 2010).
In a laboratory testing of steel slag (type unknown) as a coarse aggregate in high strength PCC, dune sand was used as the fine aggregate along with a cement to water
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ratio of 0.35 and a cement content of 759 lbs./yd3 (450 kg/m3) with a coarse to fine aggregate ratio of 1.63. The type of weathering of the slag was not stated in this study.
A naphthalene-based super plasticizer was used in the mixture to improve the workability of the mixture. This test compared three different limestone aggregates (calcareous, dolomitic and quartzitic), and one steel slag aggregate. The 28 day compression strengths of the specimens were found to be 6200 psi (43 MPa) for the calcareous specimen, 6500 psi (45 MPa) for the dolomite aggregate specimen, 6800 psi (47 MPa) for the quartzitic specimen. The highest was the steel slag specimen at 7800 psi (54 MPa), (Beshr et al., 2003).